1. Field of the Invention
The present invention generally relates to a safety treatment for the ends of metal beam guardrails. The invention more particularly relates to a mechanical energy absorbing system that is used as a terminal at the end of a metal beam guardrail, especially a box beam or W-beam guardrail.
2. Description of the Prior Art
Along most highways there are hazards that can be a substantial danger to drivers of automobiles which leave the highway. To help reduce such danger, guardrails have been provided. The guardrails are typically installed along roadsides or medians such that the upstream ends, or terminal ends of the guardrails facing the flow of traffic, do not in themselves form a hazard. Early guardrails lacked any special termination at the upstream ends, and occasionally impacting vehicles became impaled on the guardrails causing intense deceleration of the vehicle and severe injury to the occupants. In some reported cases, guardrails have actually penetrated directly into the occupant compartments of the vehicles with fatal results.
Upon recognition of the need for proper upstream guardrail termination, guardrail designs have been developed that use box beams and W-beams which allow tapering of the end of a guardrail into the ground, thereby providing a ramp and eliminating spearing effects. While these end treatments have successfully reduced the danger of a vehicle being penetrated in a head-on collision, it has been discovered that these end treatments tend to induce launching of a vehicle, causing it to become airborne for a considerable distance with the possibility of roll over.
In search for solutions to these problems, improved end treatments such as break away cable terminals (BCT), vehicle attenuating terminals (VAT), and the Sentre end treatment have been developed. All of these end treatments are designed to avoid the dangerous vaulting and roll over of vehicles. Thus, the BCT end treatment is designed to cause a W-beam guardrail to buckle out of the way of an impacting vehicle. However, since this design relies on dynamic buckling of the W-beam, it is sensitive to many installation details such as barrier flare rate and end off-set. Consequently, this design has not had a generally favorable service history.
The conventional BCT requires its W-Section to have the integrity and rigidity to redirect vehicles which impact downstream of the end. In order to maintain this beam and tension member strength, however, the resulting column strength is too large for small vehicles or those with notable soft zones in the front end unless a major eccentricity is built into the system as by flaring the end away from traffic. While such flaring has been advocated by the AASHTO 77 Barrier Guide it often has not been done in practice. It appears that this is one reason why many vehicles continue to be speared on BCT's every year.
The VAT safety treatment consists of overlapped guardrail sections that have a series of closely spaced slots. The guardrail segments are attached by bolts extending through the slots. When a vehicle impacts the end of this barrier, the bolts are forced to tear through the W-beam from one slot to the next. As a result, W-beam segments are cut into several long ribbons as an impacting vehicle is decelerated. Accordingly, the VAT safety treatment is, for all material purposes, a single-use system which has proven costly because of the need to replace the treatment after each collision.
The Sentre end treatment is constructed from a series of break away steel guardrail posts and fragile plastic containers. Impacting vehicles are decelerated as the guardrail posts are broken and as sand bags in the plastic containers are impacted. A cable is used to guide vehicles away from the guardrail during impact. This system is very expensive, and due at least in part to this expense, has not gained wide acceptance.
There is therefore a continuing need for an inexpensive, reusable end treatment which can be used at the end of a guardrail as a means of attenuating head-on impacts, and which will maintain structural integrity during impacts to longitudinal sides of the barrier. It is also desirable that such an end treatment be easily installed as an original installation or as a retrofit for existing terminals of other designs. It should also be easily serviced and inexpensive to manufacture.